US 7692612 B2 Abstract In visual display devices such as LCD devices with backlight illumination, the backlight typically consumes most of device battery power. In the interest of displaying a given pixel pattern at a minimized backlight level, the pattern can be transformed while maintaining image quality, with a transform determined from pixel luminance statistics. Aside from, or in addition to such minimizing, a transform also can be used for image enhancement, for a displayed image better to meet a visual perception quality. In either case, the transform preferably is constrained for enforcing one or several display attributes.
Claims(30) 1. A computer method for generating a pixel display pattern from a pixel given pattern using a nonlinear continuous transform from given pixel values to generated pixel values, the method comprising:
(a) for at least one display attribute, determining for each one of the given pixel values, via a processor, constraints which include an upper bound and a lower bound on the slope of the transform at each respective one of the given pixel values, wherein each lower bound is greater than 0, and wherein the constraints for at least one of the given pixel values differ from the constraints for at least one other of the given pixel values; and
(b) for a background having a background luminance value and subject to the said constraints, determining the transform via the processor such that, if the pixel display pattern is displayed against the background, visual distortion as compared with the pixel given pattern is minimized.
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local statistics of the given pixel values in a histogram bin spanned by the at least one of the linear pieces, and
global statistics over all histogram bins.
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29. A device for generating a pixel display pattern from a pixel given pattern using a nonlinear continuous transform from given pixel values to generated pixel values, the device comprising:
(a) a first device portion which, for at least one display attribute, is made for determining for each one of the given pixel values, via the first device portion, constraints which include an upper bound and a lower bound on the slope of the transform at each respective one of the given pixel values, wherein each lower bound is greater than 0, and wherein the constraints for at least one of the given pixel values differ from the constraints for at least one other of the given pixel values; and
(b) a second device portion which, for a background having a background luminance value and subject to the said constraints, is made for determining the transform via the second device portion such that, if the pixel display pattern is displayed against the background, visual distortion as compared with the pixel given pattern is minimized.
30. A processor for generating a pixel display pattern from a pixel given pattern using a nonlinear continuous transform from given pixel values to generated pixel values, the processor comprising:
(a) a first processor module which, for at least one display attribute, is instructed for determining for each one of the given pixel values, via the first processor module, constraints which include an upper bound and a lower bound on the slope of the transform at each respective one of the given pixel values, wherein each lower bound is greater than 0, and wherein the constraints for at least one of the given pixel values differ from the constraints for at least one other of the given pixel values; and
(b) a second processor module which, for a background having a background luminance value and subject to the said constraints, is instructed for determining the transform via the second processor module such that, if the pixel display pattern is displayed against the background, visual distortion as compared with the pixel given pattern is minimized.
Description Related to the present patent application is patent application Ser. No. 11/496,191 of Jul. 31, 2006. The invention concerns power management and image enhancement in visual display devices and, more particularly, in liquid-crystal display devices. Visual display devices are ubiquitous in battery-powered portable electronic devices such as notebook computers and mobile, hand-held telephones where, typically, they are the largest consumers of battery power. For example, in mobile devices equipped with thin-film transistor (TFT) liquid-crystal displays (LCD) utilizing backlight illumination, the LCD panel consumes more than 30% of the device power and the backlight typically consumes 75% of the LCD power. Thus, for conserving battery power, there is primary interest in minimizing the power consumption of the display device. An LCD screen typically includes an array of liquid-crystal pixels arranged as a plurality of rows each having a plurality of pixels, arranged in columns, with each pixel capable of displaying any one of 256 luminance values of a gray scale and the corresponding chrominance values. Each pixel has its own liquid crystal cell, a dedicated thin-film transistor, and a dedicated capacitor. The electrical field of the capacitor controls the orientation of the liquid crystals within the cell, determining the optical transmissivity of the cell and thus its luminance when lit by a backlight. The capacitor is charged and discharged via its transistor. Device activation typically is row-by-row, so that, at any one time, all column lines are connected to a single row. For saving power in an LCD device, dynamic backlight control can be used, involving dynamic scaling down and up of the backlight brightness while the device is being used, e.g. in playing back a movie. Moreover, it is beneficial to correspondingly transform an image/pattern to be displayed by transforming the pixel luminance values. When the display backlight is set at a specific brightness value, a preferred transformation, (1), of the pixel values can be determined for minimizing perceived image distortion between the original untransformed image at maximum backlight and the transformed image under the specific backlight condition. Furthermore, a preferred transformation, (2), of the pixel values can be determined for minimizing power consumption while meeting an image-quality requirement. A preferred transformation can maximize the luminance of a given pattern and provide optimal contrast by assigning each pixel a value from a given dynamic range of pixel values based on the value of the probability density of the pixel luminance values for the given pattern. Preferably, in effecting a transformation, certain display attributes are taken into account for imposing constraints on transform parameters. Transform Shape For a transformation technique, Constraints On Transform Shape A preferred transform will meet certain constraints for preventing undesirable effects. For example, if at any stage the slope of the transform were 0, then all the pixel values in that range would get compressed to a single value, resulting in total loss of contrast there. This is the case for certain transforms previously known in the art which clip the high pixel values to a threshold value, resulting in washout of bright pixels in the image. For example, washout will impair images of bulbs and lights due to loss of contrast in the region of the light. Conversely, if a slope is too large, pixel values that are close to each other are dramatically separated in the pixel-value space, causing a distorted rendition as compared with the original image. For example, where a given pixel value distribution has a peak, a swath of pixels have very similar luminance, e.g. in the court of a basketball scene. If the transform then has a high slope for these pixel values, the transformed court will have dramatically varying luminance, and the image will appear distorted. In addressing such concerns, a preferred technique imposes two constraints on the slope, m, of the transform in any stage:
With these constraints, our preferred multistage transform can give good power savings through dynamic backlight control, maintaining brightness, maintaining contrast where necessary, avoiding contrast distortion in important regions, and minimizing washout effect as compared with known transforms. Estimation of Multistage Transform For the present description of a preferred technique for estimating a desired multi-stage transform we assume that u is given, i.e. that we know the required backlight scaling factor, and hence the power savings. The technique aims at finding a transform that minimizes distortion while achieving power savings determined by u. A basic exemplary procedure can be described as follows: (a) In regions where the histogram value is high, i.e., where there are several pixels with that range of values, we maintain the original contrast by using as large a slope value as possible. (b) In regions where the histogram value is low, i.e., where there are few pixels with values in that range, we use a lesser slope. Contrast is reduced in these regions, but because it affects only a few pixels the reduction is not perceived as much. We start by dividing the x-axis into B bins, where B can be between 0 and 255. A typical value of B may be between 5 and 30. Then we integrate the original histogram within these bins to get a piecewise uniform density function as shown in
On taking account of the constraints given by Equation 1, we obtain the following for determining the slope for bin i:
Typically, x For task (2) we further seek to meet a prescribed maximum perceived brightness after backlight scaling. 1. Apply MULTISTAGE alone. Use the maximum value, t(255), to determine the backlight scaling factor, t(255)/255. This seeks to give the best possible image without trying to meet any particular power saving goal. 2. Apply MULTISTAGE. Determine the backlight scale factor based on the desired backlight setting, u. The scale factor is given by u/255. Then apply SCALING. This yields an optimal video or image and also meets the desired power saving goal. 3. Apply a transform with a fixed slope from 0 up to a certain threshold pixel value. Use MULTISTAGE after this threshold value. This seeks to maintain maximum brightness, while still achieving contrast at the high pixel values. 4. Proceed per Application 3 above, and then apply SCALING as in Application 2. 5. Proceed per Application 4 above, but, when applying SCALING, scale the fixed-slope transform only if the minimum slope constraints cannot be met. 6. When used for video, apply a low-pass filter in time to smooth the transform determined for each frame of a scene by any of Applications 1-5 above. This minimizes flicker as may result from very fast transform changes frame to frame. 7. Apply a high-pass filter to sharpen the edges of the video processed by any of applications 1 to 6 described above. -
- 8. By scaling of chrominance pixels, apply a color boost to the chrominance values for improving the color combination of the processed image. For example, in a preferred embodiment in the YUV space, the U and V components each are scaled up by a respective fixed factor. Alternatively, chrominance scaling can involve a generic functional transform of the luminance component.
Interaction With Environment For an over-all view of a typical implementation of our technique, Techniques of the invention can be applied for static backlight setting of an individual display, or dynamically in a scenario where the backlight can be changed from frame to frame of a video sequence. In either case, in processing a frame, the processor Technological Benefits And Uses Techniques of the invention can generate high-quality video, still images, graphics, and screen shots of other multimedia applications such as Microsoft Power Point and Word applications, all at minimized display backlight power or at any specific display backlight power. Furthermore, the techniques can be useful for enhancing a display even where there may be little or no concern with backlight power. Our techniques can be implemented for power management and/or image enhancement in notebook-PC's, media players such as DVD playback devices, handheld consumer electronic devices, portable media players, personal digital assistant (PDA) devices, LCD TV's and mobile phones, for example. Patent Citations
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